1. Field of the Invention
The present invention relates to an antilock brake control system for a vehicle, including normally-opened solenoid valves and normally-closed solenoid valves corresponding wheel brakes, and a diode capable of exhibiting a function of slowly decreasing electric current supplied to a coil of each of the normally-opened solenoid valves when the supply of the electric current to the coil has been cut off.
2. Description of the Related Art
There is an antilock brake control system for a vehicle already known, for example, from Published Japanese Translation No. 10-504259 of PCT Application No. PCT/DE/00588 and the like, in which a diode is connected in parallel to a coil of each of normally-opened solenoid valves in order to the suppress the generation of a noise upon closing/seating of each of the normally-opened solenoid valves.
The diode is adapted to slowly decrease the electric current flowing through the coil when the supply of the electric current to the coil has been stopped. When each of the normally-opened solenoid valves is adapted to be controlled in a turning-on/off manner, and it is also controlled at an electric current having a middle value between a turning-on value and a turning-off value, it is possible to stabilize the middle current value. However, when the control mode is changed from a turned-on state to a middle current value state, the change in electric current is slow, so that the response is retarded. Therefore, in a technique disclosed in the above Published Japanese Translation No. 10-504259, when the control mode is changed from the turned-on state to the middle current value state, it is changed from the turned-on state via a turned-off state to the middle current value state. However, the response is inevitably retarded, because the turned-off state exists in the middle.
There is also a conventionally known antilock brake control system for a vehicle, in which braking fluid pressures are controlled independently for front wheels and rear wheels. In such an antilock brake control system, a braking load on the side of the front wheels becomes larger than that on the side of the rear wheels due to a change in load during braking. Thus, it is desired that the responsiveness during the antilock brake control is higher on the side of the front wheels than that on the side of the rear wheels.
Further, there is an antilock brake control system for a vehicle already known, for example, from Published Japanese Translation No. 2000-504291 of PCT Application No. PCT/DE97/02783 and Japanese Patent Application Laid-open No. 2001-48000, in which select-low controls are carried out simultaneously for left and right rear wheels in order to ensure the stability of the vehicle during an antilock brake control.
If a capacity of a diode is set at a relatively large value, there is an effect in respect of the stabilization of an electric current. However, if the select-low controls are carried out simultaneously for the left and right rear wheels, an imbalance may be generated in some case between braking pressures of the left and right rear wheel brakes due to a difference between the capacities of diodes individually corresponding to the left and right rear wheels and a difference between inductances of coils individually corresponding to the left and right rear wheels. Moreover, there is a possibility that the imbalance is generated significantly by a reduction in responsiveness due to a larger capacity of the diode.